This invention relates to a bicycle sprocket drive assembly such as is typically used in a chain driven bicycle. The assembly consists of: right and left crank arms, on which the pedals mount; a crank shaft connecting the crank arms (or the unitary combination of those elements in a one-piece structure); and the front or drive sprocket of the bicycle. This front sprocket is engaged by the crank arm-crank shaft combination so that it is driven by the force applied to the pedals.
More particularly, this invention relates to a structure that provides the driving engagement between the crank arm-crank shaft combination, hereinafter referred to as the crank, and the front or drive sprocket.
The structures used previously to drive front sprockets from a crank included sprocket engaging shapes associated with the crank which were polygonal, splined, or some other non-rotatable shape. The sprockets received those shapes in cut-through axial mating apertures or in mating shapes machined or cast into the sprocket face or hub.
Those prior art structures using cut-through aperatures all suffer from the possibility of early failure of the drive by failure of the material around the central hole of the sprocket. For example, it has been found that the splines on sprocket assemblies which use the mating spline arrangement tend to tear away from the sprocket after long use when high driving forces are applied to the pedals. Such failures are in the nature of fatigue failures at the high stress regions of the sprocket's spline structure. The prior art structures which use machined or cast shapes have the disadvantage of being expensive to manufacture in that more metal is required for the sprocket along with more expensive machining or casting procedures.
In addition to problems with material failures and the more expensive manufacturing processes required in the prior art arrangements, they have generally required that the parts have closer tolerances and that they be more carefully aligned during assembly than is desired for ease and economy of manufacture.
It is an object of this invention to provide a sprocket driving arrangement which will be more durable than the prior art arrangements by being less subject to fatigue failure while requiring less metal in the parts.
It is another object of this invention to provide a sprocket driving arrangement which will provide increased ease and economy of manufacture along with greater durability.